Alkylene carbonates, such as ethylene carbonate and propylene carbonate are widely used as solvents and diluents in industrial processes. They are regularly used as raw materials for commercial products such as cosmetics and pharmaceuticals. Alkylene carbonates can also be used as intermediates in the preparation of alkylene glycols from alkylene oxides.
Alkylene carbonates are produced commercially by the reaction of carbon dioxide with the appropriate alkylene oxide. In the art, ionic halides, such as quaternary ammonium halides, quaternary phosphonium halides and metal halides, are frequently proposed as catalysts for this reaction.
According to JP-A-57,106,631, the preparation of alkylene carbonate as an intermediate in the two-step preparation of alkylene glycol can occur by the reaction of an alkylene oxide with carbon dioxide in the presence of an alkali metal halide.
U.S. Pat. No. 4,314,945 is directed to the preparation of an alkylene carbonate by reaction of the corresponding alkylene oxide with carbon dioxide in the presence of a catalyst characterized by the formula M+A−, wherein M is potassium and A is iodine or M is a quaternary ammonium cation (R1R2R3R4N+) and A is either bromine, chlorine or iodine. The reaction is carried out in alkylene carbonate.
U.S. Pat. No. 4,786,741 is directed to the reaction of alkylene oxides with carbon dioxide in the presence of a catalytic composition and water. Catalytic compositions listed include organic quaternary ammonium halides, organic quaternary phosphonium halides, organic sulphonium halides and organic antimony halides.
JP-A-59,013,741 teaches a method for producing ethylene glycol from ethylene oxide via ethylene carbonate. The reaction of ethylene oxide with carbon dioxide to form ethylene carbonate is catalysed with a quaternary phosphonium halide.
Quaternary phosphonium and ammonium halides are known to degrade under reaction conditions suitable for the carboxylation of alkylene oxides. This can lead to contamination of the product stream with degradation products and hence a reduction in the purity of the required product. In the case of quaternary ammonium halides the degradation products can include amines, which are stench compounds and may be detected by their odour at very low levels of contamination (e.g. ppm or ppb levels).
There are several examples in the prior art of the combination of metal halides with polyethers such as polyethylene glycol and crown ethers as a catalytic composition for the carboxylation reaction of alkylene oxides to alkylene carbonates.
The fixation of atmospheric carbon dioxide, as propane-1,2-diol carbonate, by reaction with 1,2-epoxypropane is taught in K. Kasuga, N. Kabata, Inorganica Chimica Acta, 257 (1997) 277. Here, a combination of sodium iodide and 15-crown-5 was found to give the highest yield of propane-1,2-diol carbonate for this reaction, when carried out in either chloroform or dichloromethane.
The experiments described in G. Rokicki, W. Kuran, B. Pogozelska-Marciniak Monatshefte für Chemie, 115 (1984) 205 are directed to the use of potassium salt-phase transfer agent systems as catalysts in the reactions of a variety of epoxides with carbon dioxide to yield the corresponding carbonates. In these experiments, the reaction is carried out in the absence of a solvent.
In W. Huang, S. Wu, et al. Fenzi Cuihua, 12 (1998) 447-452, the use of cycloethers containing metallic iron is compared with the use of di-, tri- and polyethylene glycols and dibenzo-18-crown-6 as carboxylation catalysts in combination with potassium iodide for the reaction of ethylene oxide with carbon dioxide in tetrahydrofuran, which has been dried prior to use.
JP-A-56,128,778 is directed to the combination of alkali metal halides with crown compounds in the preparation of alkylene carbonates. The preparation reaction is again carried out in the absence of a solvent.
The explosion potential of epoxides is well known. Therefore, the use of such compounds undiluted would be unfavourable on a commercial scale.